1. Technical Field
The present invention relates to polymer solid electrolytes, methods of production thereof, and lithium ion secondary batteries.
2. Related Art
Along with the development of portable electronic devices such as portable personal computers and video cameras, higher energy density and longer charge and discharge cycle are needed for secondary batteries used for these devices. There are also increasing demands for safety arising from the need to install the secondary batteries in medical electronic devices and in automobile equipment that becomes high temperature.
Lead batteries, nickel-cadmium batteries, and nickel-hydrogen batteries are known examples of secondary batteries widely used in electronic devices, and these batteries are installed in a wide range of devices from automobile equipment to personal portable electronic devices. In recent years, lithium ion secondary batteries have been put to actual applications and are widely used as secondary batteries having higher voltage and higher capacity density than other common batteries.
An electrolyte used for the secondary batteries is typically an acidic or alkaline aqueous solution, and the lithium ion secondary batteries use an organic electrolytic solution to avoid reaction between lithium and water. However, these liquids are dangerous because a leakage causes damage to the device or chemical damage to the human body, or may lead to accidents such as smoking and fire. It is therefore necessary to be considerate of design and environment to avoid leakage. Further, in the lithium ion secondary batteries, repeated charge and discharge cycle causes deposition of needle-like lithium metal (dendrites). This may damage the internal structure of a cell and cause shorting, which may pose serious danger such as heat generation and bursts. There have been attempts addressing these issues and concerns.
For example, JP-A-2006-261024 discloses a gel electrolyte obtained by preparing a mixed solution of a 1:1 (molar ratio) mixture of a borate ester of diethylene glycol monomethacrylate and a borate ester of triethylene glycol monomethyl ether, a 1:1 (volume ratio) mixed solvent of ethylene carbonate and diethyl carbonate, LiBF4, and a polymerization initiator, passing the solution between a pair of stainless steel electrodes, and maintaining the solution in a sealed container at 65° C. for 2 hours.
For example, JP-A-2009-9703 discloses an ion conductive solid electrolyte film obtained as follows. A 2-cyanoethylacrylate monomer (70 g), a polymerization solvent (acetone; 163 g), a radical initiator (2,2′-azobisisobutyronitrile), and a chain transfer agent (lauryl mercaptan) are allowed to react to one another. A LiClO4 acetone solution is then mixed with poly(2-cyanoethylacrylate) obtained after precipitating, purifying, and drying the reaction mixture, and the resulting mixture is dried under reduced pressure after removing the solvent.
The gel electrolyte disclosed in JP-A-2006-261024 still inherently involves the risk of fire, because the fire or burst may occur as the solvent leaches out under heat or increased pressure, or needle-like dendrites may develop and damage a separator to generate heat or cause fire. The organic solid electrolyte disclosed in JP-A-2009-9703 is also problematic, because it has low ion conductivity at ordinary temperature and may require a thin layer thickness or a temperature at or above a softening point, or may cause shorting due to a rupture resulting from insufficient strength of the thin layer, or may generate the dendrites during the charge and discharge under heat.